Trip Log:
In many of our earlier rides it felt like we could climb steeper hills if we
had sufficiently lower gears. Because most inexpensive mountain bikes come
with rear cassettes that have only a 28 tooth low gear the motor is only
driving at 2:1 (14T sprocket on the motor to 28T cog on the rear wheel)
To get much lower gearing we decided to build a bike using the SRAM DualDrive
rear hub. This hub has 3 internal gears and also has a 7,8 or 9 speed cassette
gear. We chose the 8 speed cassette for a total of 24 powered gears. The DualDrive
hub's 3 internal gears are designed to give you roughly the same gear ratios
as you would have with the standard 44/32/22 chainrings up front. The hub has
a 1:1 normal ratio, a 1.36:1 high range and a 0.73:1 low range.
By using a Mega-Range 11/32 rear cassette and shifting the Dual Drive into the low
range, we effectively now have a gear ratio of 14:32 * 0.73 or 3.13:1. That's much
lower than our original 2:1 ratio. The overall effect is that we can now let the
motor purr away as we go up much steeper hills.
In our early tests we've been able to climb 6% grades at 10-12mph without pedaling.
We've also gone up an 8% grade at about 8.5mph without pedaling. On this hill we
were in 2nd gear and only pulling 450 watts - 10% below our motor's rating of 500w
and way below the 950 watts we've pulled with this motor under other test
circumstances.
With some pedaling, this bike will go up the 8% grade at over 12mph. Due to time
and weather limitations, we have not yet tested on steeper hills but the preliminary
indications are that this setup will be a real hill killer. What's more, on our
speed tests, we topped out at 31.6mph with pedaling and 30.2mph without pedaling.
Conclusions:
The Li-Ion battery, 500w motor and the DualDrive rear hub are a great combination on
hills.
An added bonus is that the 3-speed hub can be shifted when stopped. This makes
taking off from stop signs a real breeze - you don't have to worry about shifting down
as you come to a stop, just click down once you're at a standstill and then shift up
through the 3 ranges to quickly get back up to full speed.
Stay tuned for full-scale hill climb tests...